Recently, the optical waveguide device has attracted notice in the field of various optical instruments, particularly optical measuring instruments for obtaining information relating to the phase and amplitude of the surface of an object to be detected. The reason resides in its advantage taht the use of an optical waveguide has the effect of reducing the size and weight of an optical system and eliminating the need to adjust the optical axis.
Depending on the difference in refractive index between the light guide core and the cladding forming the optical waveguide, the waveguide width, the refractive index distribution, etc., the various optical waveguides are divided into the following classes, that is, the single-moded waveguides in which only the 0-order moded beam is excited, the double-moded waveguides in which the 0-order and first-order moded beams are excited and the multi-moded waveguides in which at least three beams of the 0-order, first-order and second-order modes or over are excited.
For instance, a confocal laser scanning microscope in which mode interference in a double-moded waveguide is utilized to obtain a differential interference image of the surface of an object to be detected, is disclosed in the paper by H. Ooki et al; "A novel type of laser scanning microscope: theoretical considerations" Optics Communications, vol. 85, No. 2, 3; September 1991, pp 177-182.
With the optical instrument described in this paper, when a laser beam is incident to the double-moded waveguide, a 0-order moded beam and a first-order moded beam are excited in accordance with the amplitude distribution of the incident laser beam. The two moded beams interfer with each other in the waveguide thus causing the light intensity distribution in the waveguide to become asymmetrical. By preliminarily choosing the length of the double-moded waveguide to be optimum and putting forth two branch waveguides from one end of the double-moded waveguide, information relating to the asymmetry of the light intensity distribution in the double-moded waveguide can be obtained by detecting the difference in intensity between the light beams emitted from the branched two branch waveguides. It is considered that the asymmetry of the light intensity distribution in the waveguide is due, on one hand, to the asymmetry of the intensity distribution of the incident laser beam and, on the other hand, to the asymmetry of the phase distribution of the incident laser beam. By optimally selecting the length of the double-moded waveguide, it is possible to detect either of the asymmetric distributions. The information relating to the asymmetry of the intensity distribution can be used in the measurement of the distribution of light reflectance on an object surface to be detected and the information relating to the asymmetry of the phase distribution can be used in the measurement of the inclination, unevenness, height or the like of the object surface.
On the other hand, a laser scanning microscope for detecting the displacement in the optical axis direction (the deviation from in-focus) of an incident laser beam by utilizing the interference of modes in a double-moded waveguide made of an optical fiber is disclosed in the paper of R. Juskaitis et al; "Surface profiling with scanning optical microscopes using two-mode optical fibers", Applied Optics, vol. 31, No. 22; August 1992, pp 4569-4574.
The optical instrument described in this paper is of the construction in which the incident position of a laser spot is off the center of the end bace of a double-moded optical fiber. Where the laser spot is out of focus, the equiphase surface of the incident light to the optical fiber is inclined. This inclination cases asymmetry to appear in the phase distribution of the incident light to the optical fiber. The deviation from in-focus (the focusing position) is measured by detecting the asymmetry of the phase distribution.
In this optical instrument using the optical fiber, its light source and the optical fiber are separately arranged and therefore the alignment of the incident light with the optical fiber is difficult. Also, due to the use of the optical fiber, it is difficult to choose the length of the double-moded waveguide to be optimum. In view of these reasons, this optical instrument cannot ensure such a high degree of measuring accuracy as expected.
Also, a confocal laser scanning microscope using an optical waveguide device to separately obtain the phase information and amplitude information of an object surface to be detected is disclosed in Japanese Laid-Open Patent Application JP-A 4-208913 publicly made open on Jul. 30, 1992.
This confocal laser scanning microscope includes a channel waveguide device at the position of a spot image formed by a condensing optical system from the reflected light from an object surface to be detected. This optical waveguide device includes a double-moded waveguide having its end face positioned at the spot image position, and a single-moded waveguide for directing the illuminating light from the light source to the optical path to the object surface through the end face. The single-mode waveguide is arranged in a positional relation such that its center is aligned with the center of the double-moded waveguide whereby due to the light incident to the double-moded waveguide from the single-moded waveguide, only the 0-order moded beam is excited and a spot illuminating light having an intensity of a normal distribution and a phase of a normal distribution is emitted from the end face of the double-moded waveguide toward the object surface to be detected.
In this case, extremely sophiscated manufacturing techniques are required for the manufacture of an optical waveguide device in which a single-moded waveguide and a double-moded waveguide are in a positional relation such that their centers are aligned with each other.